Mechanisms of Instability in Ru-Based Dye Sensitization Solar Cells

In-situ infrared studies performed with operating Ru-complex-sensitized wet solar cells using a total reflection technique reveal that the ruthenium c...
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J. Phys. Chem. B 1997, 101, 2564-2575

Mechanisms of Instability in Ru-Based Dye Sensitization Solar Cells R. Gru1 nwald and H. Tributsch* Hahn-Meitner-Institut, Department Solare Energetik, Glienicker Strasse 100, 14109 Berlin, Germany ReceiVed: August 14, 1996X

In-situ infrared studies performed with operating Ru-complex-sensitized wet solar cells using a total reflection technique reveal that the ruthenium complex (both tri- and mononuclear) attached to TiO2 is photoelectrochemically transformed and irreversibly consumed under conditions of insufficient regeneration by iodide or from the oxide within the nanocrystalline TiO2 pores. The sensitizer [(Ru(bpy)2(CN)2)2Ru(bpca)2]2- (bpy is 2,2′-bipyridine, bpca is 2,2′-bipyridine-4,4′-dicarboxylate) decomposes into fragments; one of them was identified to be Ru(bpy)2(CN)2. For the sensitizer Ru(bpca)2(SCN)2, it is shown that a molecular fragment (absorbing at 2013 cm-1) is generated which is diffusing out of the nanostructured TiO2 layer. Due to its correlation with the photocurrent density, it is identified as a product of the oxidized sensitizer. Due to a high serial resistance introduced by the total reflection element and the resulting low fillfactor of the sensitization cell during in-situ measurements, only small photocurrents (5-10 µA cm-2) could be passed through the sensitizing interface. Since the rate of product formation should be proportional to the ratio of photocurrent density to iodide concentration, the iodide concentration was correspondingly reduced (1-10 mM) as compared to the conditions in a solar cell (10 mA cm-2, 1 M). This spectroscopic technique was developed because efforts to produce stable sensitization solar cells proved to be unsuccessful due to sealing problems. Our experiments do not seem to permit extrapolation to 107-108 electron transfer numbers for sensitizing Ru complexes, and real long-term testing is required for reevaluating long-term performance.

Introduction A. Energy Conversion via Sensitization Cells. The history of sensitization of large-gap semiconductors for visible light is more than hundred years old and closely associated with the development of photography.1-3 Only after the basic principles of semiconductor electrochemistry were understood in the 1960s systematic studies of this phenomenon were started with sensitized oxide electrodes in electrochemical cells.4-7 Relevant research on the mechanisms involved were performed in the laboratory of professor H. Gerischer, to whom this contribution is dedicated. The first demonstration that the sensitization phenomenon can be used to convert light into electrical energy in a sensitization solar cell has been given in 19728 (or previously, but less accessible, in 1968 by the same author in ref 9). The sensitization solar cell with chlorophyll as the sensitizing dye and ZnO as the large band-gap oxide has been described as a model system for the primary process in photosynthesis where excited chlorophyll molecules are injecting electrons into electron transfer protein chains.10 During the following two decades, more then 200 publications appeared which were relevant for the photoelectrochemical energy conversion in sensitization solar cells (e.g., refs 11-26) or which tried to explain the underlying electron injection mechanisms (e.g., ref 27). Among the papers dealing with energy conversion, there were several attempts to reach reasonably high energy conversion efficiencies. Among these the work of Tsubomura et al.,11 of Matsumura et al.,15 and Alonso-Vante et al.17 should especially be mentioned. They reached between 1% and 2.5% energy conversion efficiency when illuminating dyes which are sensitizing ZnO ceramics in their absorption region. By that time, quantum efficiencies for electron injection per absorbed photon approaching 100% have been reported and confirmed even for the same dye system, for example, ZnO in the presence of rose bengal and the reducing agent hydroquinon.9,28 Also, X

Abstract published in AdVance ACS Abstracts, March 15, 1997.

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the favorable role of a large oxide surface has been recognized and best results were reported with porous ceramic electrodes.29 Many research groups, however, earlier or later discontinued the research on sensitization solar cells, because of problems with the stability (see below). Around 1990, a significant progress regarding the efficiency of dye sensitization cells was reported by M. Gra¨tzel and collaborators.30 They used TiO2 in the form of carefully prepared nanocrystalline layers, in combination with organic electrolytes (acetonitrile and ethylene carbonate), and ruthenium complexes as sensitizers (Figure 1). An important requirement was also the attachment of the ruthenium complex to the TiO2 surface by carboxyl groups, a technique previously developed by Dare-Edwards and others.18 The original publication in Nature30 has been widely discussed in the popular press and in popular scientific journals.31-33 The main attention was attracted by the information that this type of solar cells costs only 10-20% of the present photovoltaic cells and that the ruthenium complex may sustain 107-108 turnovers of electron injection. While the high energy efficiency reported by the research group in Lausanne has been confirmed,36 (in the range of at least 7-10%), long term stability measurements under solar intensity by independent groups have not been documented. Several groups, including our own, have also tried in vain to obtain cells for long-term testing. We know of no independent research group that has succeeded in maintaining a stable sensitization solar cell in full solar light with high efficiency for a longer time period (more than one year). Our own experience with the nanocrystalline cell proposed by Gra¨tzel started from the time immediately after the publication of the article in Nature30 when we attempted reproducing these results. B. Stability Information from Early Sensitization Solar Cells. Already, after the first publication in which solar energy conversion via dye sensitization cells has been proposed and evaluated, the problem of instability has been addressed, © 1997 American Chemical Society

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Figure 1. (A, top) Structure of dye sensitization cell with indication of critical parameters for longterm stability: 1, Sealing against evaporation of electrolyte; 2, instability of electrolyte (e.g., thermolysis); 3, photoelectrochemical degradation of sensitizer; 4, photochemistry of I-/I2 system, sensitizer, and electrolyte; 5, TiO2-catalyzed photooxidation of sensitizer and electrolyte; 6, chemical reactivity of iodine with sealing material. (B, bottom) Atomic force microscopic picture of a nanocrystalline TiO2 layer. The grayscale in the bar on the right indicates the height profile.

though the system was considered to be a “promising new type of photoelement”: “the experience has shown, that photo electrochemical side reactions accumulate products which may be responsible for the gradually increasing contamination of the electrode surface during a long duration of operation. Considerable research will be needed to control unwanted side effects”. In preceding research,9 a gradual photodecomposition of sensitizing molecules was observed when light was passed

through platelets of zinc oxide to excite dyes at the oxideelectrolyte interface. It was observed that a colored spot developed where the light was penetrating the oxide. A redcolored spot (in the case of rodamine B) developed, because water-insoluble products accumulated at the oxide interface and permitted adsorbtion of dye molecules from the electrolyte. The comparison of absorption spectra of electrolytes before and after a long-term photo current flow showed a significant change due

2566 J. Phys. Chem. B, Vol. 101, No. 14, 1997 to the accumulation of reaction products, i.e., for the system ZnO in contact with rose bengal and hydroquinon. Side reactions were also evident in the case of chlorophyll a sensitizing ZnO in the presence of hydroquinon. It was observed, that after chlorophyll a should have been consumed 20 times over (after a period of approximately 14 h), the photocurrent density only amounted to approximately 20% of the initial value.10 In this study, it was clearly demonstrated that the sensitizing molecules occupy a wide spectrum of energetically different sites, which react with different kinetic constants. This means that there are adsorbed molecules which are easily destroyed irreversibly, and there are others which are not and, therefore, serve as sensitizers for the electron transfer for extended time periods. Another study of a sensitization solar cell from 19761 emphasized not only the favorable role of a large oxide surface but also the essential role of iodide as a reducing agent for the regeneration of the sensitizing dye. In an early study of TiO2 sensitization with ruthenium complexes bound to titanium dioxide by two ester linkages, Dare-Edwards and collaborators observed the deterioration of the sensitization phenomenon after the illumination of the electrode for many hours in absence of a reducing agent.18 From these early studies it appears to be clear that sensitizing molecules undergo a risk of being oxidized. Reducing agents decrease this risk on the basis of their ability to regenerate the dye molecules or to avoid the oxidation by forming a charge transfer complex which finally facilitates the electron injected into the oxide. However, the stability of a dye sensitization cell during a long-term operation (many months to years) has never been reported before the Lausanne group. This group was, however, the first to apply extensively Ru complexes, which allow metal-centered electron transfer reactions, for sensitization at TiO2 interfaces. This reported stability of Ru complexes seems to be in contrast to their behavior in homogeneous environment. While electron transfer is expected to occur mainly from a charge transfer state having largely triplet character (3CT), thermal activation to an upper d-d state may occur, which leads to significant distortion along the Ru-Nbonding axes with chemical consequences such as ligand loss or ligand reorganization.38 In homogeneous solutions, Ru complexes may typically only sustain 20-100 electron transfer processes before irreversible transformation. In order to serve as sensitizers for commercial dye sensitization solar cells, the Ru compounds must behave significantly more stable when chemically bonded to the TiO2 interface. This is, in fact, what has been claimed for the Ru-based sensitization cell and will be reinvestigated in this study. The question also arises whether an increase of the effective oxide surface by improving the nanostructure of the material can be the reason for an increased stability of the sensitization cell. When better chemical adsorption sites are available, higher electron transfer rates to TiO2 may be obtained, which more successfully compete with thermally activated d-d states. Previously reported sensitization solar cells (with 2.5% energy efficiency for incident, monochromatic light) have also been optimized with respect to high porosity or surface area (aluminum-doped zinc oxide).15 Their preparation as oxide ceramics from commercially available fine structured and further ground powder may, after sintering, also have led to “nanocrystalline” oxides. Therefore, the difference between the stability behavior of oxide ceramics and nanocrystalline oxide layers may be expected to be only a gradual one. The aim of our contribution is to bring attention to the problems of instability, since stability is a basic requirement

Gru¨nwald and Tributsch for solar cells, namely, their price and their practical applicability for energy conversion. Experimental Section A. Cell Preparation. TiO2 films were prepared according to published procedures:44 12 g of TiO2 (Degussa/Germany, P25) was ground in a porcelain mortar with 4 mL of H2O containing 0.4 mL of acetylacetone to prevent reaggregation of the particles. H2O (15 mL) was slowly added 1 mL at a time; finally 0.2 m of Triton X 100 (Fluka) surfactant was added to facilitate spreading of the nanocrystals onto the substrate. Regular solar cells were made on 8 Ω cm SnO2/F glass substrates. Cells for infrared measurements were prepared on a silicon substrate (see below). The edges of the substrate were covered with Scotch tape (40 µm thick). P25 colloid (5 mL cm-2) was applied and evenly distributed using a glass rod. The film was fired in a stream of air for 30 min at 450 °C. In some cases, the films were treated with 50 µL cm-2 of 0.2 M solution of TiCl4 in H2O and again fired. For dye sensitization, the films were removed still hot (80 °C) and dipped into 4 × 10-4 M dye solution (see part D) in dry ethanol. Electrolyte was 0.5 M tetrapropylammonium iodide (TPAI)/ 0.04 M I2/0.02 M KI in acetonitrile 20% and 80% propylene carbonate for long term experiments. For in-situ IR experiments the electrolyte was 0.5 M tetrapropylammonium perchlorate (TBAP) in propylene carbonate with variable amounts of KI or 0.5 M Na2SO4 + 5 mM hydrochinone in aqueous solution (for the detection of CO stretch vibrations). Counter electrode for the two electrode sandwich cells was a platinized 8 Ω cm SnO2/F glass plate. The structure of the sensitization cell together with weak points concerning long term stability to be investigated in this study are shown in Figure 1A. Figure 1B is an atomic force microscopic image visualizing the size of the nanocrystals and of the pores. B. Stability Experiments. After the initiated long-term experiments with dye sensitization cells (6 cells to be tested in parallel for a period of 1 year) turned out to be impossible due to the rapid evaporative loss of the toxic organic electrolytes, efforts concentrated on improving the sealing techniques. Additional experiments were performed in dessicators saturated with electrolyte vapor. Experiments were also performed during which the lost electrolyte was regularly replaced to permit the continuation of experiments with the dye sensitization cells. C. In Situ Infrared Spectroscopic Experiments. In situ infrared spectroscopic measurements were made in an attenuated total reflectance (ATR) geometry on a Bruker IFS 113v FT infrared spectrometer equipped with standard components (Globar, KBr beamsplitter and IN2-cooled mercury-cadmiumtelluride detector). The electrochemical cell, made from plastic material (vespel, which has a high chemical stability similar to Teflon), is shown in Figure 2. It has a regular three-electrode setup with a Pt wire as the counter and a homemade Ag/AgSO4 reference electrode. Experiments in the nonaqueous solution (propylene carbonate) were performed in a two-electrode configuration. The working electrode was a TiO2 film coated on a trapezoidal silicon crystal (50 × 20 × 2 cm, doping density (n) 1015 cm-3) used simultaneously as the ATR infrared window. The electrical contact was made by applying a Ga/In eutectic to the backside of the crystal. The electrode current or potential was controlled by a standard galvanostat/potentiostat (HEKA electronics). The high internal resistance of the cell introduced by the low-doped Si crystal and possibly a thin insulating SiO2 layer formed during heat treatment of the TiO2 films constrained the current level which could be driven through the cell. The

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Figure 2. Setup for total reflection in-situ FTIR spectroscopic measurement of the Ru complex in the dye sensitization solar cell: sideview (left), frontview (top right), topview (bottom right). (CE, RE, WE, counter, reference, and working electrodes (TiO2-coated Si crystal); CS, contact and spacer (copper); W, window; I/O, inlet/outlet for the electrolyte solutions; D, IR detector.)

potential drop at the internal resistance had also to be considered when applying an external potential to the solar cell. Illumination with visible light was done with a tungsten/iodine source with a 400 nm cutoff UV filter and a water filter with a 13 cm path length to remove a large part of the infrared output of the lamp. Test measurements with a xenon high-pressure lamp and no UV filter confirmed that photocatalytic reactions initiated by valence band holes did not contribute to the observed IR signals in the time window of the experiments (30-60 min). The area of the working electrode in contact with the solution was 7.36 cm2 allowing for 12 useful reflections at the Si/porous TiO2 interface. The evanescent wave, decaying exponentially, has a penetration depth into the TiO2 layer of about 2 µm in the spectral range studied (1500-4000 cm-1). This is equivalent to the region probed by the IR radiation with the highest sensitivity immediately at the Si/TiO2 interface. The thickness of the TiO2 layers prepared from Degussa P25 material was typically 10 µm unless otherwise stated. During measurements, the interferometer space was evacuated and the sample compartment purged with dry N2 to secure stable and low levels of water vapor and CO2 in the path of the infrared beam. For each spectrum, 256 scans were coadded to increase the signal to noise ratio; resolution was 4 cm-1. The interferograms were then Fourier transformed to give the spectra. Further processing of the data included dividing by a reference spectrum and base line correction. The spectra are presented as the conventional ∆R/R, relative change in IR reflectance, scaled to one reflection with signals pointing up meaning a loss and signals pointing down creation of IR-absorbing species. The area under the bands in the ∆R/R vs wavenumber plot gives the integrated IR signal. The solutions were made from ultrapure water coming from a Millipore system or alternatively propylene carbonate (p.a. Aldrich) which was dried over a molecular sieve. The solutions were purged with argon prior to use and kept under a slight overpressure of Ar during the experiments. All other chemicals (tert-butylammonium perchlorate (TBAP), hydrochinone, Na2SO4 and KI) were at least reagent grade and used without further purification. D. Ruthenium Complexes. The trinuclear Ru complex with pendant bisbipyridyl units (referred to as Ru3 complex), as used in ref 30 was obtained from Prof. Scandola. After a Ru complex with improved efficiency was adopted for the sensitization

Figure 3. Infrared spectrum and molecular structure of investigated Ru complexes: (A) Ru3 complex, (B) mononuclear Ru complex.

cells42 also this compound, cis-dithiocyanato-N-bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II), was studied (in the following named Ru complex). It was synthesized following published procedures.43,44 Stoichiometric amounts of RuCl3 (Fluka) and 2,2′-bipyridyl-4,4′-dicarboxylic acid (bpca, Aldrich) were refluxed under Ar in DMF for 8 h to give Ru(II)(bpca)2Cl2, which was precipitated with acetone. After they were cleaned, dried, and filtered, the crystals obtained were used as starting material for synthesis of Ru(bpca)2(SCN)2. They were dissolved in a DMF/H2O mixture with appropriate amounts of NaSCN in basic conditions and refluxed 6 h in Ar atmosphere. After this, the solvent was removed on a rotary evaporator and the resulting solid was washed and filtered several times. The molecular structures of the ruthenium complexes together with the measured infrared spectra are shown in Figure 3A,B. For example, S- and N-coordinated thiocyanide ligands are identified (at 1998 and 2107 cm-1, respectively) in the spectrum of Ru(bpca)2(SCN)2. (Compare Figure 3b with refs 34, 35, and 44).

2568 J. Phys. Chem. B, Vol. 101, No. 14, 1997 Results A. Long-Term Stability Studies: The Problem of Liquid Confinement. Long-term experiments with TiO2 sensitization cells, as described by Gra¨tzel et al.30 were initiated in which it was envisaged to operate 6-8 cells in parallel in order to get statistical information on stability problems. It can, however, not be claimed that the chemical conditions were exactly identical. Typically a slightly different electrolyte composition (propylene carbonate instead of ethylene carbonate) was used. Soon it was learned that long-term experiments are not possible because the cells were rapidly drying out. Experiments with sensitization cells (sealed, using a Wacker silicon rubber and colorless E43) were then initiated using a glass container in which they were either exposed to regular air, or alternatively to saturated vapor of the same electrolyte used in the cells, or refilled by adding electrolyte to the drying cells. Small cells (1 cm2) sealed with a simple silicon glue dried out within 2 days. If they were exposed to saturated vapor of the same electrolyte used in the cell, this evaporation could be reduced and the cell sustained 150 h of illumination during a period of 1 month. When the evaporated liquid was replaced in certain time periods, the photoelectric output of the cell could be maintained over extended periods. In the beginning, significant efforts were put into overcoming these trivial problems by making the strip of glue between the glass plains broad or by introducing additional barriers, i.e., aluminum foil. Also, increasing the dimension of the dye sensitization cell makes the ratio of rim length to light exposed cell area smaller (stability increases proportional to the dimension). By using glues with improved properties for this purpose (e.g., Epox Seal, Donelley Corporation) or by adapting sealing techniques elaborated by the liquid crystal industry, the effective sealing of the liquid sensitization solar cells can significantly be improved so that stabilities of weeks or months are obtained. (Recently the DuPont hot-melt glue Surlyn is being used by other groups.) However, as we learned from the photographic industry,39 a serious problem remains. It is even today very difficult to seal liquids with ambient temperature sealing techniques with the aim of preventing their evaporation. The reason why Polaroid film material cannot be guaranteed for much more than one year is basically a sealing problem. Even though polymer coated aluminum foils and intricate additional techniques are applied, this slow evaporation cannot be stopped even at the moderate temperature at which photographic material is typically kept. Sealing the dye sensitization system entirely into glass is of course an alternative (glass soldering). However, such a step, which requires elevated temperatures, is not inexpensive and requires some technical refinement (it may be anyway a precondition for reliable long-term cell testing). B. Problem of the Electrolyte. When operating a sensitization solar cell based on ref 30, gas bubbles may occasionally appear in the electrolyte volume. This is carbon dioxide which originates from thermolysis of ethylene carbonate or propylene carbonate, a phenomenon which has been known for a long time. The decomposition process involved makes a long-term operation of the sensitization cell as described in ref 30 impossible. For this reason, alternative electrolytes (N-methyloxazolidinone and imidazol salt electrolytes) have been proposed by the Lousanne group. But also in this case electrolytic components are lost through evaporation so that long-term experiments cannot easily be made. There is not sufficient experience to verify that any one of these electrolytes can be used for long-term solar cell operation.

Gru¨nwald and Tributsch An additional problem is the I-/I3- electrolyte, which is not only quite reactive toward sealing glues, but may itself undergo photochemical reactions, so that long-term testing is required. The role of the electrolyte in the dye sensitization process is complex. Both the photovoltage, and thus the energy conversion efficiency, and the cell stability depend on the electrolyte composition. Typically high efficiency (organic electrolyte) and relative stability (inorganic electrolytes) are not achieved with the same electrolyte. In addition, organic electrolytes contain, even after intensive cleaning treatment, at least 4-10 ppm of water, the concentration of which gradually increases due to the imperfect low-temperature sealing. In our spectroscopic in-situ studies of sensitizing ruthenium complexes both organic and aqueous electrolytes were used. No significant difference was observed in the short-term irreversible degradation process. Since the carboxylate bands, which reflect the binding of the Ru complex to the TiO2 interface, are well visible in presence of an aqueous electrolyte, IR spectra taken under such conditions were relevant for our studies. For this reason the behavior of the Ru3 complex during in-situ studies is discussed with aqueous medium and that of the mononuclear Ru complex with organic medium (see below). C. In-Situ Infrared Studies of the Ruthenium Complex. According to Gra¨tzel, the dye sensitizer has to be considered the weakest point in the sensitization solar cell. Hence, it has to be insured that it could work effectively for at least 10 years without degradation.34 The Ru(bpca)2(SCN)2 complex was indeed reported to have sustained more than 5 × 107 redox cycles during prolonged illumination without noticable loss of performance corresponding to approximately 10 years of continous operation in natural sunlight.40 However, no experimental details were given in ref 40. A method which has been used previously comprising illuminating the cell with chopped light from an intensive laser and counting the photons assuming that each one initiates an electron injection process equivalent to a regular electron transfer event in a solar cell is, in the following, argued to be inappropriate. The first detailed study on the stability of the ruthenium complex in which the 2,2-bipyridine, 4,4-dicarboxylic acid is chemically attached to the semiconductor surface by two ester linkages has been performed by Dare-Edwards et al.18 In absence of a reducing agent they observed a gradual decrease of the sensitization current by one order of magnitude within 2 h. This degradation could be stopped by the presence of a super sensitizer (1,4-dehydroxybenzene). Our aim was to study the reaction mechanism of ruthenium complexes, when sensitizing TiO2 interfaces, with the described infrared technique. To our knowledge this study represents the first in-situ vibrational study of the sensitized wide band gap semiconductor electrolyte interface under conditions of solar cell operation. As shown in Figure 4, due to the high internal resistance of the silicon total reflection element including a possible oxide layer, the measured photocurrents were quite low (1-10 µA, cm-2, two different cell characteristics shown, Figure 4B). For comparison, solar cells prepared with the same methods on conducting fluoride doped tin oxide coated glass showed photocurrent densities of typically 1-5 mA cm-2 (Figure 4A). It should be emphasized that the deterioration of the solar cell fill factor when connecting the sensitization cell in series to the high resistivity silicon total reflection crystal does not imply any change of interfacial mechanisms. It only means that a substantial part of the generated photopotential drops across the additional resistance. Since a clear turnover of the Ru complex was observed with the infrared technique even under conditions of low current

Ru-Based Dye Sensitization Solar Cells

Figure 4. Typical photocurrent power output characteristics of both dye sensitization cells, (A) Ru3 complex, electrolyte was propylene carbonate:acetonitrile 8:2; 0.5 M LiI, 0.04 M I2. (B) Effect of high internal resistance of the total reflection element. Mononuclear Ru complex, electrolyte was propylene carbonate, 0.5 M TBAP, 5 mM I-; illumination was 100 mW cm-2 (tungsten/iodine lamp, 400 nm cutoff filter, IR water filter).

efficiency, the phenomenon of irreversible degradation is significant. Figure 5A shows infrared spectra obtained with the total reflection setup for in-situ measurement for different positive electrode potentials (indicated in the legend) under illumination. The absorption band developing in upward direction near 2100 cm-1 corresponds to a loss of Ru3 complex (ex-situ spectrum plotted in Figure 5A at the bottom). The absorption band developing in downward direction near 1995 cm-1 corresponds to a product. Since the loss spectrum near 2100 cm-1 shows an additional structure not seen in the exsitu spectrum (minima at 2075 cm-1 and less clearly at 2050 and 2095 cm-1) additional product bands which are superposed onto the loss band may be present (with higher current density and with increasing photocharge passed the features of these structures become increasingly more pronounced). The in-situ behavior of the Ru3 complex is shown here in contact with aqueous medium to expose the infrared spectra down below 1500 cm-1 (buried in presence of organic electrolyte). They are presented in Figure 5B. Again an in-situ spectrum (spectrum a) is seen, which depicts the change in absorption after 58 mC cm-2 photocharge have been passed through the cell in reference to the spectrum at open circuit potential (also illuminated). It is compared with the ex-situ IR spectrum in the same spectral region. Figure 5B, spectrum a) shows the superposed loss band (peaks) and product bands (minima). The loss maxima at 1539 and 1593 cm-1 correspond well to similar peaks in the absorption spectrum (Figure 5B, spectrum b). Consequently, the minima at 1549 and 1624 cm-1 can be identified as IR absorption bands of photoelectrochemically generated products. Interestingly, an additional loss signal (upwards) is seen at 1670 cm-1, where the H2O combination band is situated. This means that the photoinduced interfacial processes are accompanied by a change in the solvation conditions. No clear upward (loss) signal is observed in the region of carboxylate vibrations (near 1615 cm-1). This means that a significant separation of the carboxylate group from the interface is not taking place as a consequence of the photoprocesses. In Figure 5C sequences of darkness and illumination (15 mW cm-2) applied to the sensitization cell were investigated. After 30 min of electrochemical polarization at +200 mV, no change of the IR spectrum, measured against the spectrum at open circuit or in darkness (they are identical) was observed. However, after illumination (10 and 16 min with decreasing photocurrent density) a clear change of the spectrum, with loss and product bands, is observed (Figure 5C, spectra b and c). After a period of darkness (30 min) an additional spectrum (Figure 5C, spectrum d) was recorded. The product peak at 1995 cm-1 has disappeared; the loss peaks are on the other hand

J. Phys. Chem. B, Vol. 101, No. 14, 1997 2569 are still present, even though somewhat smaller. This means that an irreversible loss of Ru3 complex from the surface has occurred. The in-situ infrared spectra of the Ru3-complex have also been investigated in presence of 5 mM KI as reducing agent. Only minor changes in position and intensity of loss and product bands occurred in this case. The photoelectrochemically induced in-situ IR absorption changes, observed with the monomeric Ru complex Ru(bpca)2(SCN)2, will now be discussed for the organic electrolyte propylene carbonate (with 0.1 M TBAP), in absence of iodide. No absorption by water could be detected in the IR spectrum. Figure 6 shows the spectra obtained when three different photocurrent densities are passed through sensitization cell. The evident peaks at 1985 cm-1 (fitted to 1988 cm-1 with a halfwidth of 29 cm-1) and 2104 cm-1 correspond to the loss of Ru(bpca)2(SCN)2, while a new product is identified at 2013 cm-1 (fitted to 2005 cm-1 with a half-width of 42 cm-1). Apparently, an additional product peak (with an absorption coefficient 5-10 times smaller) is located near 2130 cm-1. It can be seen that there is no turnover in absence of a photocurrent, that it is increased with the density of the photocurrent, and that the turnover (loss and product) increases with the photocharge passed through the interface. The loss peaks at 1988 cm-1 (corrected) and 2104 cm-1 show in the in-situ spectrum a half-width of 29 and 26 cm-1, respectively, which is smaller than the half-widths of 33 and 32 cm-1, respectively of the corresponding peaks in the ex-situ spectrum. Presumably not all Ru(bpca)2(SCN)2 molecules contribute equally to the photoelectrochemical process and their kinetic behavior depends on the nature of the adsorption sites. Molecules which do not inject electrons and dissipate the excitation energy or which are rapidly regenerated do not contribute to the in-situ IR signal. In Figure 6 it is seen that a decrease of photocurrent reduces the loss of sensitizer in an unproportional way. Below (for comparison), the time dependent spectrum for the case of zero photocurrent has also been recorded (illumination period 70 min). This shows that the turnover of sensitizer requires the passage of photocurrent. Figure 7 shows the effect of addition of iodide to the electrolyte. Comparing sensitizer loss and product accumulation at a photocurrent density of 4.8 µm cm-2 in Figure 6 and Figure 7 clearly shows that already the presence of 1 mM KI markedly reduces the turnover of the sensitizer. At 10 mM KI, however, the sensitizer turnover is still detectable after passage of only 14 mC cm-2. This result allows a simplified extrapolation. With a 200 times larger photocurrent density (∼1 mA cm-2) and a correspondingly 200 times higher KI concentration (2 M) and a passed photocharge of 2.8 C cm-2 the turnover should still be detectable. If 10 mA cm-2 of photocurrent would be passed, as under solar cell operation conditions, the turnover of sensitizer would increase at least correspondingly. Figure 8 shows the influence of photocharge passed on sensitizer loss and product generation. It can be seen that during the galvanostatic experiments a limiting concentration of reaction products is finally approached. While immediately after an increase of photocurrent density loss bands (1985 and 2104 cm-1) and product band (2013 cm-1) significantly increase, the increments subsequently gradually decrease and finally level off. However, at higher photocurrent density (5.7 µA cm-2), the signals at 2104 and 2013 cm-1 continue to increase. There is consequently no direct proportionality between IR signal and charge turned over at the site of sensitization. There are two possibilities: a further reaction product is formed with a negligible IR signal or we are dealing with a mobile reaction

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Figure 5. Evolution of IR spectra during in-situ measurement of the trinuclear Ru complex in the dye sensitization cell. (A) In-situ IR spectra of a Ru3-complex-sensitized TiO2 film in the CN strech region under illumination. Spectra a: potentiostatic control, +100 mV, 22 µA cm-2; illumination period, 10 min. Spectra b: +200 mV, 68 µA cm-2; illumination period, 19 min. Spectra c: potentiostatic control, +300 mV, 124 µA cm-2, illumination period, 24 min. Spectra d: potentiostatic control, +500 mV, 135 µA cm-2; illumination period, 33 min. Spectrum under open circuit in the dark was used as the reference. (Electrolyte: 0.5 M Na2SO4 + 5 mM hydrochinone in aqueous solution. Illumination 15 mW cm-2 open circuit potential: +200 mV in the dark, +50 mV under illumination vs Ag/AgSO4.) The IR absorption spectrum of the dye is also shown (bottom). (B) In-situ IR spectrum of a Ru3-complex-sensitized TiO2 film in the CO stretch region under illumination. Potential, +0 V; photocharge, 58 mC cm-2. Spectrum under open circuit under illumination was used as the reference (a). (Electrolyte: 0.5 M Na2SO4 + 5 mM KI in aqueous solution. Illumination, 15 mW cm-2, open circuit potential, -330 mV in the dark, -700 mV under illumination vs Ag/AgSO4). The IR absorption spectrum of the dye is also shown (b). (C) In-situ IR spectra of a Ru3-complex sensitized TiO2 film in the CN stretch region; potential fixed at +200 mV: (a) dark, 1 µA cm-2; (b) illuminated, 96 µA cm-2; illumination period, 10 min; (c) illuminated, 73 µA cm-2; illumination period, 16 min; (d) dark, 0 µA cm-2. Spectrum at +200 mV in the dark was used as the reference. Spectra have been shifted vertically for clarity. Other conditions as in Figure 5A. The IR absorption spectrum of the dye is also shown (bottom).

product, which diffuses out from the IR total reflection volume (∼2 µm into the porous TiO2 layer). Figure 9 explains experiments aimed at identifying the reason for reaching a limiting concentration of products within the TiO2 pores. After passing a defined photocharge, the electric circuit was interrupted (open circuit) and the integrated IR signal of loss (at 1985 and 2104 cm-1) and product bands (2013 cm-1) followed in time. The right-hand axis gives the decrease of the 2104 cm-1 band relative to the state of the surface before the charge was passed (100% means the complete disappearance of the band; 0% indicates that the band is unaffected by the photoelectrochemical experiment). The 2104 cm-1 band has been selected because no superposition with a strong downward band interferes with the determination of the integrated intensity as is the case with the 1985 cm-1 band. In Figure 9A it is seen that, in absence of iodide, the product at 2013 cm-1 disappears after approximately 2000 s. At the same time the accumulated loss peaks at 2104 and 1985 cm-1 again decrease by approximately 15 and 35%, respectively (relative to the value at t ) 0 in the Figure 9A). However, an irreversible loss is

maintained. In presence of 10 mM KI, the behavior is similar, but decrease of losses and product is more pronounced (∼4045%) (Figure 9B). The time constant for the decay is still approximately 2000 s. In Figure 9C a sensitized TiO2 layer is investigated with a thickness of only 1 µm in absence of KI. The time constant for the disappearance of the loss peak (2013 cm-1) in this case only amounts to 400 s. It is seen that, while the magnitude of recovery of losses is strongly dependent on the iodide concentration, the typical time constant is dominated by the thickness of the porous TiO2 layer. The irreversible loss is significant amounting up to 20% in the experiment of Figure 9B in the presence of 10 mM iodide. It should be pointed out that the ATR infrared technique is only probing a narrow reaction zone close to the back-contact of the nanocrystalline TiO2 layer. This is the region where iodide regeneration via diffusion is most inefficient. Several experiments have been performed to identify the product peak at 2013 cm-1. Cerium(IV) sulfate, which has a clearly more positive redox potential than the Ru complex (Eo ) +1.09 V) was added to a an acid solution of Ru(bpca)2(SCN)2

Ru-Based Dye Sensitization Solar Cells

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Figure 8. Integrated in-situ IR signal (sensitizer loss was 1985 and 2104 cm-1 and product accumulation was 2013 cm-1) in dependence of passed photocharge at indicated photocurrent densities. (Electrolyte: 0.1M TBAP in propylene carbonate, illumination